1. Technical Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus and an image forming method, and particularly to an image reading apparatus having a monochrome reading function to read a color document as a monochrome image, an image forming apparatus and an image forming method.
2. Related Art
Heretofore, there is a light source switching type color image reading apparatus (for example, a color scanner) or color image forming apparatus (for example, a color copying machine), in which irradiation light sources of plural colors, for example, irradiation light sources of red (R), green (G) and blue (B) are switched to read a color document, and a color image is formed.
In the color image reading apparatus or the color image forming apparatus, a monochrome reading mode is often provided in which a color document is read as a monochrome image and the monochrome image is formed.
On the other hand, by the realization of a blue LED (Light Emitting Diode) with high light emitting efficiency, a mode in which all of three primary colors of red (R), green (G) and blue (B) are constructed of LEDs has become possible. By constructing all irradiation light sources by the LEDs, as compared with a conventional structure using a fluorescent lamp or a cold-cathode fluorescent lamp, the increase in life, improvement in reliability, reduction in size and weight, and the like become possible.
However, in the case where monochrome reading is performed using, for example, only a blue LED, there arises a problem that a letter of blue ink is hard to reproduce. Besides, in the case where monochrome reading is performed using only a red LED, an image of a seal of a vermilion inkpad is hard to reproduce. This phenomenon occurs especially remarkably in a light source with a narrow light emission distribution, such as an LED, and it may cause a problem when reading depending on the color of the document.
The reason why the read output of a document varies according to the color of the document is mainly caused by color sensitivity (sensitivity to light wavelength) determined by the spectral distribution of a light source and the spectral distribution of the document. In an extreme case, there is a case where a specific color on the document can not be read at all, and this specific color is called a dropout color.
In order to solve this problem, a patent document (JP 11-341222 A) discloses a technique in which three LED light sources of red (R), green (G) and blue (B) are used to perform monochrome reading.
The technique disclosed in this patent document 1 is the technique in which the three LED light sources of red (R), green (G) and blue (B) are made to produce a composite output by multiplication of a fixed coefficient ratio (or a coefficient ratio obtained by finely adjusting this) so as to coincide with a human visual sensitivity.
According to the technique disclosed in this patent document, when a limitation is made to only the letter reproduction by various color inks, there is a possibility that the object of eliminating the dropout color can be achieved.
When the monochrome reading of a color document is performed, in addition to the simple elimination of the dropout color, there is a user's request that the density (or brightness) of a reading output be changed according to the kind of color.
For example, in the case where a document includes a blue sky and a red letter, there is a case where it is desired that the sky does not become very dark but is as bright as possible, while the red letter is clearly (rather dark) outputted. Besides, in the case where the document includes a person and a blue letter, there is a case where it is desired that the flesh color of the person is as bright as possible, while the blue letter is clearly (rather dark) outputted.
As stated above, in the case where a document includes plural colors and it is desired that those colors are outputted with different densities (or brightnesses), it is difficult to realize by the composition of the outputs of the three LEDs with the fixed coefficient ratio as disclosed in the patent document.
Besides, the patent document discloses that matching with the human visual sensitivity is performed with the ratio coefficient multiplied to the outputs of the three LEDs. However, since the human visual sensitivity and the LED spectral distribution are originally significantly different from each other, it is conceivable that the description disclosed in the patent document only is insufficient for the achievement of the object to achieve the matching with the human visual sensitivity.
In addition, there has also been a problem that when an original document is irradiated by a light source in which LED elements of plural colors are arranged in an array shape, uneven density occurs in the reading density of a color document because the irradiation beams of the plural colors do not sufficiently overlap with each other on the document.